What does this PR do?

Adds an HTTPX2Wrapper backed by httpx2 behind a use_httpx2 instance config flag. The default remains RequestsWrapper. Also widens the OpenMetrics v2 scraper to catch HTTPConnectionError alongside requests.exceptions.ConnectionError.

Motivation/Why

Design validation for the migration RFC. httpx2 is the Pydantic-maintained successor to httpx. The goal is to land on a maintained library before any integration opts in.

Approach

• New HTTPX2Wrapper sits alongside RequestsWrapper in datadog_checks/base/utils/.
• AgentCheck.http dispatches to HTTPX2Wrapper only when the instance sets use_httpx2: true. Otherwise it constructs RequestsWrapper exactly as before, so the default path is untouched.
• The wrapper exposes a library-agnostic response adapter and maps httpx2 exceptions onto the shared http_exceptions hierarchy. This translation runs only on the new engine's side: RequestsWrapper still raises native requests exceptions, so callers cannot yet rely on a single neutral contract regardless of backend. The OM v2 scraper works across both clients because it was explicitly taught to catch the requests and httpx2 connection errors side by side. Full exception-contract unification across both backends is follow-up work, not part of this PR.
• The shared HTTPResponseProtocol is widened with encoding (read + setter), url, cookies, and elapsed, plus @runtime_checkable, so both wrappers' response adapters expose the same surface. Existing callers that wrote through response.encoding are now protocol-covered.
• The OM v2 scraper's connection-error handler is widened to (ConnectionError, HTTPConnectionError) so a streamed scrape fails the same way under either client.

Verification

• New unit suite under tests/base/utils/httpx2/ covers verb dispatch, config/TLS/auth propagation, exception mapping, lifecycle/dispatch, the response adapter, and the OM scraper connection-error paths.
• ddev test -fs datadog_checks_base passes (format + style clean).
• Full datadog_checks_base base suite green except pre-existing kerberos/socks5 integration tests that require Docker (unrelated to this change).
• Default RequestsWrapper behavior is unchanged: no integration sets use_httpx2, so production callers are unaffected.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add qa/required if this PR needs QA validation, or qa/skip-qa if it does not. Exactly one of the two is required.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged